Filters are widely applied in mobile communications systems. For example, a duplexer of a transceiver module of a base station is formed by radio frequency (RF) cavity filters, which are located on a structural part on the back of a transceiver board and are configured to transmit a single channel of high-power radio frequency signal. The cavity filter generally uses a coaxial filter with a coaxial cavity.
As mobile communications technologies develop toward multiband, a coaxial filter gradually fails to meet the requirement of mobile communications systems for low insertion loss. The emergence of a traverse electric wave 01 (TE01) mode dielectric filter with a dielectric cavity solves this problem. The TE01 mode dielectric filter is formed by a TE01 mode dielectric resonator working in a TE01 mode. Since the TE01 mode dielectric resonator is made by using high power factor (Q) microwave dielectric ceramic, the TE01 mode dielectric filter has relatively low insertion loss and meets the requirement of mobile communications systems for low insertion loss, and therefore has been developed for commercial use.
However, it is found in the process of using the TE01 mode dielectric filter that the distance between a high-order harmonic wave and a passband of the TE01 mode dielectric filter is approximately 200-400 megahertz (MHz); therefore, suppression on a high-order harmonic wave at a far end of the TE01 mode dielectric filter is less than 70 decibels (dB), and performance of far-end suppression is relatively undesirable. For a dielectric filter, the far end refers to a frequency that exceeds a high frequency end of above 100 MHz of a passband.
To correct the foregoing defects of the TE01 mode dielectric filter, two solutions are provided in the prior art. One solution is to add a metal resonator at a port of the TE01 mode dielectric filter. This solution can increase the distance between a high-order harmonic frequency and a passband, but suppression on a high-order harmonic wave still cannot meet the requirement and insertion loss is relatively high. The other solution is to add a low-pass filter at the front of the TE01 mode dielectric filter. This solution can increase suppression on a high-order harmonic wave of a relatively high frequency. However, a stopband of the low-pass filter is smooth and is not steep, and therefore, for a high-order harmonic wave at a distance of approximately 200-400 MHz from the passband, relatively high suppression cannot be obtained and insertion loss is relatively high.